In swash plate compressors such as used in vehicle air conditioning systems, the drive arrangement between the swash plate and the pistons normally comprises a ball mounted in a socket in each piston and also in a socket in a shoe having a flat side that is slidably engaged by one side of the swash plate. However, it has also been proposed to employ a semi-spherical shoe that combines these parts. Typically, the pistons are made of aluminum, the balls and semi-spherical shoes are made of steel and the shoes are made of brass. For proper operation, the fit between the piston, ball, shoe (or semi-spherical shoe), and swash plate must be held very close. In practice, this has been accomplished by machining, gauging, and sorting the shoes into certain dimensional increments or classes. For example, these increments may be as small as 0.0002" and extensive in number depending on the manufacturing tolerances. In assembly, the balls are assembled in the pistons and the gap between them is measured. The thickness of the swash plate is also measured and with these two dimensions the proper class shoe is then selected for each piston. The manpower and equipment required to sort, gauge and select are major manufacturing costs. Furthermore, the inventory of shoes must be kept high to maintain a sufficient number of parts in all the classes to meet anticipated requirements. There is also an additional problem with the steel semi-spherical shoes in that it is far more difficult in mass production to produce and maintain tolerances of a flat-sided ball as compared to a simple round ball.
Then in the field there may occur such problems as noise and smearing by the brass shoes. Noise is attributed to loss of fit from either misassembly, wear, or the steel balls coining into the aluminum pistons. When this occurs, the loose assembly will be slapped during compression rather than riding on the swash plate. And this situation will not improve but will degenerate. Smeared brass on the other hand occurs during a dry start-up when no lubrication is present between the swash plate and the shoes resulting in brass from the shoes being deposited on the steel plate and forming a brass-on-brass interface with a potential for galling.